The present invention relates in general to an electric power machine provided with a safety mechanism against overspeed, particularly an angle grinding tool having a centrifugal severing plate for severing an electric conductor in the power supply circuit of a driving motor when an acceptable rotary speed of the grinding spindle is exceeded.
Known power machines of this kind are equipped with an electronic rotary speed regulating circuit. In the event of a failure in the regulator, however, the rotary speed of the motor shaft may rise above the acceptable maximum limit. This shortcoming is particularly dangerous in the case of angle grinders because the grinding disc mounted on the spindle may break when its circumferential speed is too high. In order to reliably avoid this danger it is known from prior art to employ a fly weight arranged in a housing on the driving shaft in such a manner that in the event of overspeed of the motor, the fly weight severes a conductor web in the power supply circuit of the motor. The disadvantage of this prior art centrifugal safety mechanism is the difficulty in setting the limit rotary speed at which the fly weight is displaced outward to interrupt the power supply circuit. In the prior art solutions this adjustment can be made only by tensioning a spiral spring on which the fly weight is suspended. Due to the limited space for the safety mechanism, a screw with an eccentric pin has been used as a support for the tensioning spring and consequently the total setting displacement for adjusting the limit rotary speed amounted only to half the rotation of this setting screw. Moreover, due to the restricted conditions the installation of this prior art overspeed protecting mechanism was extremely difficult and time consuming. In particular, the suspension of the tensioning spring on the eccentric pin of the setting screw was very difficult. Furthermore, the fly weight in severing the electric conductor might become damaged itself and the end of the tensioning spring might spring out from the protective housing and reestablish the electric connection in the power supply circuit.